Allow reading/replacing mailmerge (fixed pull 1050)

[korvinko]

Description

Simplified version #1050

• Cleanup unused code
• Added setValues() method for changing preset value MERGEDFIELD
• Allow UTF8 symbols

Example of using merge:

        $templateProcessor = new MailMergeTemplateProcessor('template.docx');
        $variables = $templateProcessor->getMailMergeVariables();
        $templateProcessor->setMergeData(['agreement_number' => 10244]);
        $templateProcessor->doMerge();
        $templateProcessor->saveAs('template2.docx');

Example of using setValues:

        $templateProcessor = new MailMergeTemplateProcessor('template.docx');
        $variables = $templateProcessor->getMailMergeVariables();
        $templateProcessor->setMergeData(['agreement_number' => 10244]);
        $templateProcessor->setValues();
        $templateProcessor->saveAs('template2.docx');

[troosan]

please fix issues and add tests